JP2019201142A - Chip resistor and manufacturing method of chip resistor - Google Patents

Abstract

To provide a chip resistor capable of improving a surge characteristic and finely adjusting a resistance value with high accuracy.SOLUTION: A chip resistor 1 includes a resistor 5 which is printed so that a first meander part 6 and a second meander part 7 are continued while nipping a rectangular adjustment part 8. By forming a first trimming groove 9 in the adjustment part 8, a surge characteristic is improved by increasing a current path of the resistor 5, a resistance value of the resistor 5 is roughly adjusted so as to approach a target resistance value. By forming a second trimming groove 10 in a region having less of a current distribution in the second meander part 7, the resistance value of the resistor 5 is finely adjusted so as to be matched to the target resistance value in accordance with a cutting amount of the second trimming groove 10.SELECTED DRAWING: Figure 1

Description

    The present invention relates to a chip resistor whose resistance value is adjusted by forming a trimming groove in a resistor provided on an insulating substrate, and a method for manufacturing such a chip resistor.

  The chip resistor includes a rectangular parallelepiped insulating substrate, a pair of front electrodes disposed opposite to each other on the surface of the insulating substrate with a predetermined interval, and a pair of opposing electrodes disposed on the rear surface of the insulating substrate with a predetermined interval. It is mainly configured by a back electrode, an end face electrode that bridges the front electrode and the back electrode, a resistor that bridges the paired front electrodes, a protective film that covers the resistor, and the like.

  In general, when manufacturing such a chip resistor, a large number of electrodes, resistors, protective coating layers, etc. are collectively formed on a large substrate, and then the large substrate is formed into a grid-like divided line. A plurality of chip resistors are obtained by dividing along (for example, dividing grooves). In the manufacturing process of such a chip resistor, a large number of resistors are formed on one side of a large substrate by printing and baking a resistor paste. However, misalignment or bleeding during printing, temperature unevenness in the baking furnace, etc. Because it is difficult to avoid slight variations in the size and film thickness of each resistor due to the influence of the resistor, the resistance value adjustment is such that a trimming groove is formed in each resistor in the state of a large substrate and set to a desired resistance value. Work is done.

  When a surge voltage generated due to static electricity or power supply noise is applied to a chip resistor with such a configuration, it will affect the characteristics of the resistor due to excessive electrical stress, and in the worst case the resistor will break down. It may be done. In order to improve the surge characteristics, it is known that if the resistor has a meandering shape (a meander shape) and the entire length is increased, the potential drop becomes gentle and the surge characteristics can be improved.

  As a conventional technique of this type, as shown in FIG. 4, after forming a resistor 102 meandering two turns between a pair of front electrodes 101 provided at both ends of an insulating substrate 100, a laser trimming method is performed at the center thereof. A chip resistor has been proposed in which one trimming groove 103 is formed to obtain a resistor 102 meandering three turns (see Patent Document 1).

  As another prior art, as shown in FIG. 5, between a pair of front electrodes 101 provided at both ends of an insulating substrate 100, a rectangular portion 102a connected to the pair of front electrodes 101 and a space between the rectangular portions 102a. There is proposed a chip resistor in which a trimming groove 103 is formed in a rectangular portion 102a at both ends after a resistor 102 composed of a substantially S-shaped portion 102b positioned at a position is printed (see Patent Document 2).

JP-A-9-205004 JP 2001-338801 A

  In the conventional technique described in Patent Document 1, since the total length of the resistor 102 is increased by using both the printing technique and the trimming process, the surge characteristics can be improved and the trimming groove 103 can be formed. Since the resistance value is also adjusted, the resistance value accuracy can be improved. However, since the trimming groove 103 is formed in a direction that narrows the cross-sectional area of the current in the resistor 102, the amount of change in the resistance value that increases with the cutting amount of the trimming groove 103 increases, and the resistance value accuracy is increased to some extent. However, the resistance value cannot be finely adjusted with high accuracy.

  On the other hand, in the prior art described in Patent Document 2, since the trimming grooves 103 can be formed in the rectangular portions 102a at both ends sandwiching the substantially S-shaped portion 102b of the resistor 102, the chip resistor described in Patent Document 1 is provided. Compared to the above, the adjustment ratio of the resistance value can be increased. However, since the trimming groove 103 is formed in the direction of narrowing the cross-sectional area of the current in the resistor 102, the resistance value can be finely adjusted with high accuracy. I can't.

  The present invention has been made in view of such a state of the art, and a first object is to provide a chip resistor that can improve surge characteristics and finely adjust a resistance value with high accuracy. The second object is to provide a method of manufacturing such a chip resistor.

  In order to achieve the first object, a chip resistor according to the present invention includes an insulating substrate, a pair of electrodes opposed to each other at a predetermined interval on the insulating substrate, and a bridge between the pair of electrodes. A chip resistor, the resistance of which is adjusted by forming a trimming groove in the resistor, the resistor is connected to the pair of electrodes, and both of these connections And a rectangular adjustment portion located between the two portions, and a continuous printed forming body, and at least one of the connection portions is a turn-shaped meandering portion, and the current path of the resistor is connected to the adjustment portion. The first trimming groove for coarse adjustment to be lengthened is formed, and the second trimming groove for fine adjustment is formed in the meandering portion. The inter-electrode direction of the pair of electrodes is the X direction, and this X direction The direction orthogonal to the Y direction The meandering portion includes an extending portion extending in the Y direction, an outer turn portion extending in the X direction and connecting one end of the extending portion and the electrode, and extending in the X direction in addition to the extending portion. And the second trimming groove extends in the Y direction with either one of the outer turn portion and the inner turn portion as a starting end position, and the second trimming groove has an inner turn portion connecting the end and the adjustment portion. It is characterized in that the tip does not reach an imaginary line connecting the outer turn part and the inner turn part with the shortest distance.

  In the chip resistor configured as described above, the first trimming groove that lengthens the current path of the resistor is formed in the adjustment unit, and the resistance value increases with the cut amount of the first trimming groove. The resistance value can be coarsely adjusted while improving the characteristics, and the second trimming groove is formed in a region where the current distribution is small in the meandering portion, whereby the resistance value can be finely adjusted with high accuracy.

  In the chip resistor having the above-described configuration, only one of the two connection portions connected to the pair of electrodes may be a turn-shaped meander portion, but both of the two connection portions are turn-shaped meander portions. In addition, it is preferable that the second trimming groove is formed in either one of these meandering portions because the entire resistance value becomes long and surge characteristics can be further improved.

  In order to achieve the second object, a method for manufacturing a chip resistor according to the present invention includes an insulating substrate, a pair of electrodes disposed opposite to each other on the insulating substrate at a predetermined interval, and the pair of electrodes. In a method of manufacturing a chip resistor in which a resistance value is adjusted by forming a trimming groove in the resistor, the resistor is connected to the pair of electrodes. And a rectangular adjustment portion positioned between the two connection portions, and at least one of the connection portions is a turn-shaped meandering portion. When the inter-electrode direction of the electrodes is the X direction, and the direction orthogonal to the X direction is the Y direction, the meandering portion extends in the Y direction, extends in the X direction, and one end of the extending portion In the X direction, the outer turn part connecting the electrodes And a first trimming groove for coarse adjustment that lengthens the current path of the resistor in the adjustment part. The adjustment part has an inner turn part that connects the other end of the extension part and the adjustment part. Then, a second trimming groove for fine adjustment extending in the Y direction is formed with one of the outer turn portion and the inner turn portion as a starting end position, and the tip of the second trimming groove is connected to the outer turn portion and the It is characterized in that it is set at a position that does not reach the imaginary line that connects the inner turn portions at the shortest distance.

  In the manufacturing method of the chip resistor including such a process, after printing and forming a meander-shaped resistor in which at least one meandering portion and the adjustment portion are continuous, the first trimming is performed to lengthen the current path of the resistor in the adjustment portion. By forming the groove, the resistance value increases with the cut amount of the first trimming groove, so that the resistance value can be roughly adjusted while improving the surge characteristics, and after the first trimming groove is formed. The resistance value can be finely adjusted with high accuracy by forming the second trimming groove in the region where the current distribution is small in one meandering portion.

  According to the present invention, it is possible to provide a chip resistor that can improve surge characteristics and finely adjust a resistance value with high accuracy.

It is a top view of the chip resistor concerning a 1st embodiment of the present invention. It is explanatory drawing which shows the manufacturing process of the chip resistor which concerns on 1st Embodiment. It is a top view of the chip resistor concerning a 2nd embodiment of the present invention. It is a top view of the chip resistor concerning a conventional example. It is a top view of the chip resistor concerning other conventional examples.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a plan view of the chip resistor according to the first embodiment of the present invention. As shown in FIG. 1, the chip resistor 1 according to the first embodiment includes a rectangular parallelepiped insulating substrate 2 and first and second surface electrodes 3 and 2 provided at both longitudinal ends of the surface of the insulating substrate 2. A front electrode 4, a resistor 5 provided on the surface of the insulating substrate 2 so as to be connected to the pair of front electrodes 3, 4, and a protective coating layer (not shown) provided so as to cover the resistor 5 Etc. are mainly composed. Although not shown, a pair of back electrodes is provided on the back surface of the insulating substrate 2 so as to correspond to the first and second front electrodes 3 and 4, and both end surfaces in the longitudinal direction of the insulating substrate 2. Are provided with end face electrodes that bridge the corresponding front and back electrodes.

  The resistor 5 is formed in a meander shape in which the first meandering portion 6 and the second meandering portion 7 at both ends are sandwiched across the adjustment portion 8 at the center. Such a meander shape depends on the printing shape of the resistor paste. It is prescribed. In FIG. 1, when the interelectrode direction of the first and second surface electrodes 3 and 4 is the X direction, and the direction orthogonal to the X direction is the Y direction, the first meandering portion 6 is an extending portion extending in the Y direction. 6a, an outer turn portion 6b extending in the X direction and connecting the lower end of the extending portion 6a and the first front electrode 3 on the left side of the drawing, and between the upper end of the extending portion 6a and the adjusting portion 8 extending in the X direction. The extending part 6a, the outer turn part 6b, and the inner turn part 6c are all set to have the same pattern width.

  The second meandering portion 7 includes an extending portion 7a extending in the Y direction, an outer turn portion 7b extending in the X direction and connecting the lower end of the extending portion 7a and the second surface electrode 4 on the right side of the drawing, and in the X direction. It has an inner turn part 7c that extends and connects between the upper end of the extending part 7a and the adjusting part 8, and the pattern width of these outer turn part 7b and inner turn part 7c is set to be the same as that of the first meandering part 6. Has been. However, the pattern width of the extending portion 7 a is set to be wider (about twice) than the pattern width of the extending portion 6 a of the first meandering portion 6.

  The adjustment portion 8 is formed in a rectangular shape that is wider than the pattern width of the first meandering portion 6 and the second meandering portion 7, and the inner side of the first meandering portion 6 is on the opposite upper side of the adjustment portion 8. The turn part 6c and the inner turn part 7c of the second meandering part 7 are connected. Then, by forming two first trimming grooves 9 along the Y direction from the upper side of the adjustment portion 8 and extending the first trimming grooves 9 into an I-cut shape to lengthen the current path of the resistor 5, The resistance value of the resistor 5 is roughly adjusted so as to approach the target resistance value. When such a first trimming groove 9 is formed in the adjustment portion 8, the resistor 5 formed in the printed shape having the two meandering portions 6 and 7 has a shape of meandering for three turns. The overall length of the body 5 can be increased.

  However, the number of the first trimming grooves 9 formed in the adjustment unit 8 is not limited to two, and may be one or three or more. In this case, the current path width of the adjustment unit 8 after the first trimming groove 9 is formed is the minimum pattern width of the current path width (6a, 6b, 6c, 7b, 7c) where the trimming groove defined by printing is not formed. If the first trimming groove 9 is formed so as to be wider, the load concentration in the pattern can be concentrated on the portion formed by printing. Therefore, even if microcracks occur in the first trimming groove 9, the resistance is reduced. The influence on the value can be reduced.

  Further, an L-cut second trimming groove 10 is formed from the upper side of the inner turn portion 7c in the second meandering portion 7 toward the inside of the extending portion 7a, and the tip of the second trimming groove 10 is the outer turn. It is set at a position that does not exceed an imaginary line E that connects the portion 7b and the inner turn portion 7c with the shortest distance. Here, the portion where the current flows most in the extending portion 7a is the imaginary line E, and the second trimming groove 10 is formed in a region where the current distribution in the second meandering portion 7 is small. The amount of change in resistance value associated with the depth of cut of 10 is very small, and the second trimming groove 10 can finely adjust the resistance value of the resistor 5 with high accuracy so as to match the target resistance value.

  The shape of the second trimming groove 10 is not limited to the L-cut, and may be the I-cut second trimming groove 10. In that case, the current path width of the extending portion 7a of the second meandering portion 7 after the second trimming groove 10 is formed is the current path width (6a, 6b, 6c, 7b) where the trimming groove defined by printing is not formed. 7c), when the second trimming groove 10 is formed so as to be wider than the minimum pattern width, the load concentration in the pattern can be concentrated on the portion formed by printing. Even if this occurs, the influence on the resistance value can be reduced.

  Next, the manufacturing process of the chip resistor 1 configured as described above will be described with reference to FIG.

  First, a large-sized substrate from which many insulating substrates 2 are taken is prepared. In this large substrate, primary division grooves and secondary division grooves extending in the vertical and horizontal directions are provided in a lattice shape, and each of the squares divided by both division grooves is a chip area. In FIG. 2, a large substrate 2A corresponding to one chip area is shown as a representative, but in reality, the steps described below are collectively performed for a large substrate corresponding to many chip areas. Done.

  That is, as shown in FIG. 2 (a), after the Ag-based paste is screen-printed on the surface of the large substrate 2A, the first and second surface electrodes 3 and 4 are paired by drying and firing. Form (surface electrode forming step). At the same time as or before or after this electrode formation step, an Ag-based paste is screen-printed on the back surface of the large substrate 2A, and then dried and fired to form a back electrode (not shown) (back electrode formation step).

  Next, as shown in FIG. 2B, a resistor paste such as Cu—Ni or ruthenium oxide is screen-printed on the surface of the large substrate 2A, dried and fired, so that both ends in the longitudinal direction are first. A resistor 5 is formed to overlap the surface electrode 3 and the second surface electrode 4 (resistor forming step). The resistor 5 includes a first meandering portion 6 connected to the first table electrode 3, a second meandering portion 7 connected to the second table electrode 4, and between the first table electrode 3 and the second table electrode 4. The first and second front electrodes 3 and 4 and the adjusting portion 8 are formed in a meander shape continuously with each other.

  Here, in FIG. 2, when the extending direction of the secondary dividing groove is the X direction and the extending direction of the primary dividing groove is the Y direction, the first meandering portion 6 includes an extending portion 6 a extending in the Y direction, An outer turn portion 6b extending in the X direction and connecting the lower end of the extending portion 6a and the first front electrode 3 on the left side of the drawing, and between the upper end of the extending portion 6a and the upper left end of the adjusting portion 8 extending in the X direction And an inner turn portion 6c for connecting the two. The second meandering portion 7 includes an extending portion 7a extending in the Y direction, an outer turn portion 7b extending in the X direction and connecting the lower end of the extending portion 7a and the second surface electrode 4 on the right side of the drawing, An inner turn portion 7c that extends in the direction and connects between the upper end of the extending portion 7a and the upper right side of the adjusting portion 8 is provided.

  Next, a glass paste is screen-printed from above the resistor 5, dried and fired to form a precoat layer (not shown) that covers the resistor 5, and then laser light is irradiated from above the precoat layer. Thus, as shown in FIG. 2C, two I-cut first trimming grooves 9 are formed in the adjusting portion 8 (first trimming forming step), and the resistance value of the resistor 5 is set to the target resistance value. Coarse adjustment to a slightly lower value. These first trimming grooves 9 are formed so as to extend in the Y direction from the upper side to the lower side of the adjusting portion 8. By forming such first trimming grooves 9 in the adjusting portion 8, the entire resistor 5 is formed. Since the current path becomes long, the resistor 5 formed in the printed shape so as to have the two meandering portions 6 and 7 at this time has a meander shape that meanders three turns. The number of the first trimming grooves 9 formed in the adjustment unit 8 is not limited to two, and may be one or three or more.

  Subsequently, as shown in FIG. 2D, an L-cut second trimming groove 10 is formed in the second meandering portion 7 (second trimming forming step), and the resistance value of the resistor 5 is set to the target resistance value. Tweak to match. The second trimming groove 10 is formed so as to extend in the Y direction from the upper side to the lower side of the extending portion 7a. Care is taken not to exceed. Here, the portion where the second trimming groove 10 is formed is a region where the current distribution in the second meandering portion 7 is small, and the region has a very small resistance value change amount per trimming amount. Thus, the resistance value of the resistor 5 can be finely adjusted with high accuracy. If the tip of the second trimming groove 10 does not exceed the imaginary line E, the shape of the second trimming groove 10 is not limited to the L cut, and may be the I trimmed second trimming groove 10.

  Next, an epoxy resin paste is screen printed from above the first trimming groove 9 and the second trimming groove 10 and heat-cured to form a protective coating layer (not shown) that covers the entire resistor 5 ( Protective coat layer forming step).

  Each process so far is a batch process for a large-sized substrate 2A for taking a large number of pieces, but in the next step, a primary break process is performed in which the large-sized substrate 2A is divided into strips along the primary dividing groove. Thus, a strip-shaped substrate (not shown) provided with a plurality of chip regions is obtained (primary division step). Next, a back electrode corresponding to the first and second front electrodes 3 and 4 is formed by applying an Ag paste to the dividing surface of the strip-shaped substrate and drying and baking, or sputtering Ni / Cr instead of the Ag paste. An end face electrode (not shown) that bridges the two is formed (end face electrode forming step).

  Thereafter, a chip break having a size equivalent to that of the chip resistor 1 is obtained by performing a secondary break process of dividing the strip-shaped substrate along the secondary dividing grooves (secondary dividing step). Finally, electrolytic plating of Ni, Au, Sn, or the like is applied to both ends in the longitudinal direction of the insulating substrate 2 of each individual chip, and the first and second exposed from the end face electrode, the back electrode, and the protective coating layer. By forming an external electrode (not shown) that covers the surface electrodes 3 and 4, a chip resistor 1 as shown in FIG. 1 is obtained.

  As described above, in the chip resistor 1 according to the first embodiment, the meander-shaped resistor 5 in which the first meandering portion 6 and the second meandering portion 7 are continuous with the rectangular adjustment portion 8 interposed therebetween is formed by printing. After that, the first trimming groove 9 is formed in the adjusting unit 8 so that the current path of the resistor 5 is lengthened to improve the surge characteristic, and the resistance value of the resistor 5 is brought close to the target resistance value. Then, the second trimming groove 10 is formed in an area where the current distribution in the second meandering portion 7 is small, so that the resistance value of the resistor can be adjusted in accordance with the cut amount of the second trimming groove 10. Since the fine adjustment can be made so as to coincide with the target resistance value, the resistance value can be adjusted with high accuracy while improving the surge characteristics.

FIG. 3 is a plan view of the chip resistor 20 according to the second embodiment of the present invention. The same reference numerals are given to portions corresponding to those in FIG.
It is.

  The second embodiment is different from the first embodiment in that the pattern width of the adjustment portion 8 narrowed by the formation of the first trimming groove 9 is approximately the same as the pattern of the first meandering portion 6. The other configuration is basically the same as that of the chip resistor 1 shown in FIG.

  That is, as shown in FIG. 3, the adjustment portion 8 printed in a rectangular shape has a meandering shape by forming one first trimming groove 9, and the pattern width of the first meandering portion 6 is W. Then, the width dimension of the adjustment portion 8 before the first trimming groove 9 is formed is about 2W. Then, the first trimming groove 9 having an I-cut shape is formed at the center of the adjustment portion 8 to perform rough adjustment of the resistance value, whereby the rectangular adjustment portion 8 becomes a meandering shape and the width dimension is about half. W.

  In the chip resistor 20 according to the second embodiment configured as described above, the first trimming groove 9 is formed in the adjustment portion 8 printed in a rectangular shape, so that the first meandering portion 6 passes through the adjustment portion 8. Since the portion of the second meandering portion 7 that reaches the inner turn portion 7c has a pattern width W that is substantially the same, the hot spots can be dispersed and the temperature of the resistor 5 can be uniformed over the entire pattern.

  In the chip resistor 20 according to the second embodiment, the number of the first trimming grooves 9 formed in the adjusting unit 8 can be two or more. In that case, the number of the first trimming grooves 9 is also possible. Accordingly, the width dimension of the adjustment unit 8 at the time of printing may be changed.

  Further, in each of the above embodiments, the second trimming groove 10 is formed from the upper side of the inner turn portion 7c in the second meandering portion 7 toward the inside of the extending portion 7a, but the tip of the second trimming groove 10 is the outer turn. The second trimming groove 10 is formed from the lower side of the outer turn portion 7b in the second meandering portion 7 toward the inside of the extending portion 7a unless the imaginary line E connecting the portion 7b and the inner turn portion 7c is shortest. May be.

  In each of the above embodiments, the second trimming groove is formed in the second meandering portion 7 connected to the second surface electrode 4 out of the pair of the first meandering portion 6 and the second meandering portion 7 that are continuous with the adjustment portion 8 interposed therebetween. However, the resistance value may be finely adjusted by forming the second trimming groove 10 in the first meandering portion 6 connected to the first table electrode 3. The pattern width of the extending portion 6a in the first meandering portion 6 is preferably set wider than the pattern width of the extending portion 7a in the second meandering portion 7.

  In each of the above embodiments, the two connecting portions of the resistor 5 connected to the first surface electrode 3 and the second surface electrode 4 are both the first meandering portion 6 and the second meandering portion 7 having a turn shape. However, any one of the connecting portions may be formed into a straight shape without being bent into a turn shape. That is, in the chip resistor 1 shown in FIG. 1, the extending portion 6a and the outer turn portion 6b of the first meandering portion 6 are omitted, and the inner turn portion extending between the first surface electrode 3 and the adjusting portion 8 in the X direction. You may make it the structure of connecting by 6c.

1,20 Chip Resistor 2 Insulating Substrate 2A Large Format Substrate 3 First Table Electrode 4 Second Table Electrode 5 Resistor 6 First Meandering Part 6a Extending Part 6b Outer Turn Part 6c Inner Turn Part 7 Second Meandering Part 7a Extending Part 7b Outer turn part 7c Inner turn part 8 Adjustment part 9 First trimming groove 10 Second trimming groove E Virtual line connecting the outer turn part and the inner turn part at the shortest distance

Claims (3)

An insulating substrate; a pair of electrodes opposed to each other with a predetermined interval on the insulating substrate; and a resistor that bridges the pair of electrodes, and a trimming groove is formed in the resistor. In the chip resistor whose resistance value is adjusted,
The resistor includes a print formed body in which a connection portion connected to the pair of electrodes and a rectangular adjustment portion positioned between the two connection portions are continuous, and at least one of the connection portions is turned. It is a meandering part of the shape,
A first trimming groove for coarse adjustment that lengthens the current path of the resistor is formed in the adjustment portion, and a second trimming groove for fine adjustment is formed in the meandering portion,
When the inter-electrode direction of the pair of electrodes is the X direction and the direction perpendicular to the X direction is the Y direction, the meandering portion extends in the Y direction and extends in the X direction. An outer turn portion connecting one end and the electrode, and an inner turn portion extending in the X direction and connecting the other end of the extending portion and the adjustment portion,
The second trimming groove extends in the Y direction starting from one of the outer turn portion and the inner turn portion, and the tip of the second trimming groove connects the outer turn portion and the inner turn portion with the shortest distance. Chip resistor characterized by not reaching the wire.   2. The chip resistor according to claim 1, wherein both of the pair of connection portions are turn-shaped meandering portions, and the second trimming groove is formed in one of the meandering portions. And chip resistor. An insulating substrate; a pair of electrodes opposed to each other with a predetermined interval on the insulating substrate; and a resistor that bridges the pair of electrodes, and a trimming groove is formed in the resistor. In the manufacturing method of the chip resistor in which the resistance value is adjusted,
The resistor includes a print formed body in which a connection portion connected to the pair of electrodes and a rectangular adjustment portion positioned between the two connection portions are continuous, and at least one of the connection portions is turned. It is a meandering part of the shape,
When the inter-electrode direction of the pair of electrodes is the X direction and the direction perpendicular to the X direction is the Y direction, the meandering portion extends in the Y direction and extends in the X direction. An outer turn part connecting one end and the electrode, and an inner turn part extending in the X direction and connecting the other end of the extension part and the adjustment part,
After the first trimming groove for coarse adjustment that makes the current path of the resistor long in the adjustment portion is formed, for fine adjustment that extends in the Y direction with one of the outer turn portion and the inner turn portion as a start position The chip trimming groove is formed, and the tip of the second trimming groove is set at a position that does not reach the imaginary line connecting the outer turn part and the inner turn part at the shortest distance. Method.

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